FreeBSD
It is derived from BSD, the version of UNIX® developed at the University of California, Berkeley.
NetBSD
NetBSD is a free, secure, and highly portable Unix-like Open Source operating system available for many platforms,
OpenBSD
The OpenBSD project produces a FREE, multi-platform 4.4BSD-based UNIX-like operating system.
The only Free UNIX I've seen, besides the offering from SUN would be the older UNIX source code released from SCO, their 'Ancient UNIX.' Not sure how much use it would be for you, but it is 'Free' UNIX.
http://public.planetmirror.com/pub/a...-unix/ancient/